1. Fie1d of the Invention
The present invention relates generally to sample concentration for analyzing volatile organic compounds in air, water and soils. More specifically, the present invention relates to a filter for a sample concentrator to reduce accumulation of foam and debris during purging of an inert gas through aqueous solution.
2. Related Art
Sample concentrators are used in purge-and-trap, headspace, and thermal desorption gas chromatography ("GC") analysis. Purge-and-trap GC technique has been used for analyzing volatile organics in water since approximately the early 1970's. In 1987 the U.S. Environmental Protection Agency ("EPA") promulgated national primary drinking water regulations for certain volatile organic chemicals ("VOCs"). The EPA also proposed maximum contamination levels for eight volatile organic chemicals. These regulations require the use of the purge-and-trap GC technique. In addition to the eight regulated volatile organic chemicals, the EPA also promulgated monitoring requirements for an additional 52 synthetic volatile organic chemicals.
The EPA has approved certain analytical methods for analyzing these 60 compounds. One of the methods is 502.2, a purge-and-trap capillary-column GC method using a photoionization detector and an electrolytic conductivity detector joined in series. A second method is method 524.2, a purge-and-trap capillary-column GC-MS method.
Purge-and-trap systems for analyzing VOCs in drinking water have been assembled from a variety of equipment typically including a purging device, trap, and desorber. These systems also are referred to as sample concentrators. The purge-and-trap system or sample concentrator interfaces to a GC capillary column, then with a photoionization detector/electrolytic conductivity detector or a mass-spectrometer. These components are interconnected via pneumatic conduits.
Highly volatile organic compounds with low water solubility are extracted (purged) from the sample matrix by bubbling an inert gas (i.e., helium or nitrogen) through a five milliliter aqueous sample. Purged sample components are trapped in a tube containing suitable sorbent materials. When purging is complete, the sorbent tube is heated and backflushed with the inert gas to desorb trapped sample components onto a capillary GC column. The column is temperature programmed to separate the method analytes which are then detected with a photoionization detector (PID) and a halogen specific detector placed in series, or with a mass spectrometer.
Tentative identifications are confirmed by analyzing standards under the same conditions used for samples, and comparing results and GC retention times. Additional confirmatory information can be gained by comparing the relative response from the two detectors. Each identified component is measured by relating the response produced for that compound to the response produced by a compound that is used as an internal standard. For absolute confirmation, the gas chromatography/mass spectrometry (GC/MS) determination according to method 524.1 or method 524.2 may be used.
As stated above, the typical purge and trap system consists of the purging device, trap, and desorber. Systems are commercially available from several sources that meet EPA specifications.
Under EPA specifications, the glass purging device must be designed to accept five to twenty-five ml. samples with a water column at least 5 cm. deep. Gaseous volumes above the sample are kept to a minimum to reduce "dead volume" effects. The purged gas passes through the water column as finely divided bubbles.
The sorbent trap is a tube typically at least 25 cm. long and having an inside diameter of at least 0.105 inches. The trap contains certain sorbent materials which the EPA has specified as 2,6-diphenylene oxide polymer, silica gel, and coconut charcoal. The EPA regulations specify the ratios of the adsorbent material. The desorber must be capable of rapidly heating the trap to 180.degree. C.
The model 4460A sample concentrator manufactured by OI Analytical of College Station, Texas, is an example of a purge and trap, or sample concentrator, device. The model 4460A is a microprocessor controlled device that stores method 502.2 and 524.2 operating conditions as default parameters. Operating conditions may be changed by the user to accommodate other types of purge and trap analysis.
In addition to purge-and-trap methods and analyses, sample concentration gas chromatography is used in headspace analysis of liquids and solids, and in thermal desorption analysis of air tube samples. Headspace and thermal desorption techniques are not only used for environmental analyses, but also for clinical and industrial applications.
Typically, a sparge vessel includes a concentrator inlet at the open end thereof. The concentrator inlet is a manifold that includes a plurality of fittings: for purge gas, for an external sampler, and for dry gas. For example, dry gas may be introduced through the manifold and directed to the trap for drying out the trap. The concentrator inlet also includes sample introducing means. Valves may be provided for selectively directing the gas or analytes from the sparge vessel or inlet fitting to an outlet fitting which is connected to a conduit leading to the sorbent trap.
As discussed above, VOCs are purged from the water sample by bubbling an inert gas, such as helium, through a 5 ml aqueous sample. Typically, the sample is contained in a glass sparge vessel.
One of the problems encountered in transferring the gas/analyte from the sparge vessel to the trap is foaming of samples. Because of the bubbling helium gas, surfactant on the sample surface may be propelled out from the sparge vessel and carried into the trap. Liquid foam and particles have deleterious effects on the trap. Therefore, it is necessary to frequently replace traps due to accumulation of foam and debris that is propelled from the sparge vessel. Eventually, the liquid foam and particles cake on the trap with adverse effects on the detection of analytes.